The nuclear power industry generates a certain amount of wet radioactive wastes, and predominant among these radwastes are ion exchange resins and filter media that are used to scrub radioisotopes from reactor cooling and waste waters. The resulting suspensions or slurries of radioactive ion exchange resin, and in some cases filter media particles, must be dewatered for safe shipping and disposal. By dewatering is meant herein the removal of water from the waste particles such that the remaining free standing water, during long-term burial, constitutes no more than 1.0% of the waste volume. 10 C.F.R. Part 61. By free standing water is meant the drainable interstitial water that freely gravity drains from a bed of particles.
Bead-type and powdered-type ion exchange resins constitute the vast majority of the waste materials that must be dewatered. Such ion exchange resins average 3800 cubic feet per year per commercial power plant and represent nearly half of the total wet wastes generated by the utilities. Lesser amounts of activated carbon and inorganic zeolite particles from radwaste treatment systems must also be dewatered prior to disposal.
Prior to 1981, when the first large-scale dewatering containers were placed into service, the aforementioned types of wet wastes were mostly solidified by, for example, admixing them with dry cement powder in disposable steel drums. However, such solidification methods have unsolved problems, including achieving structural integrity, void spaces above the solidified block in a corrodible container, waste parts that are not fully encapsulated, and pasty or unsolidified materials. The pertinent relationships between waste media shape, size, chemical reactions, full-scale thermal effects, and waste media structure remain unsolved for the solidification of radioactive wastes in a container over the three hundred year design life of the storage regimen.
The driving factor behind the recent use of waste dewatering is economics. The availability of landfill disposal sites is clouded with political uncertainty, and the transportation costs to the few available disposal sites can be expected to increase with each new regulatory overlay. The result is the need for more waste-volume efficient methods of disposal or on-site storage, and in this regard dewatering processes are most attractive. Dewatered wastes need not undergo the volume expansion that solidification technologies require: instead of adding solid material to physically or chemically entrap or react with the water within the container, the water is removed from the container. Additionally, the dewatering process requires less plant floor space, capital investment, and no dusty, corrosive, or hazardous chemicals. The main mitigating circumstances against waste dewatering in the past have been changing regulations and operational uncertainty regarding the degree and amount of residual free standing water left in the container. Such free standing water is a potential vehicle for isotopic leaching, should the container fail or be punctured during transport, storage, or burial.
Prior to the free standing water criteria specified by the State of South Carolina in 1980, dewatering containers were simply thin gauge carbon steel liners with some cartridge filters unscientifically placed on the bottom. The 1980 free standing water criteria quickly illustrated a lack of understanding of the dewatering mechanisms because the containers, dewatering tests, and procedures changed rapidly. Bead resin containers were designed with conical bottoms and low point drains or suction configurations. A diaphragm pump was typically used to remove free standing water. Powdered resin containers were designed with several levels of cartridge filters.
It is expected that the use of resin dewatering will increase due to a number of reasons. Many plants are finding it is more cost effective to not regenerate their deep bed condensate polisher resins, and instead they directly dispose of the resins after one use. A significant increase in bead resin volumes per plant results. Bead resin volumes are also increasing due to the use of portable demineralizers in place of evaporators. The use of powdered resins is increasing due to closer attention to power plant water chemistry. Powdered ion exchange resins are increasingly being mixed with fibrous filter aids to help alleviate resin intrusion into the reactor cooling water.
Prior testing and certification procedures have been based upon representative waste media and have not considered the range of waste forms that occur in the field, nor the permanent storage conditions. Prior dewatering methods did not lend themselves to defined endpoints: the duration of the pumping cycle was simply extended until a subjective empirical endpoint, e.g., no apparent leakage from a punctured representative container, was observed. Thermodynamic considerations, such as condensing cycles within the container during transport, storage, or burial, have not previously been addressed. Nor have chemical form effects been addressed. An understanding of dewatering mechanisms leading to the production of consistent results has not been developed or achieved. In at least one case, an extrapolation of free standing water versus drainage time has been made using specific test results. This method was mathematically unsound and unrepresentative of the actual variety of waste forms. As a result, some of the liners punctured during field tests and at burial sites have been found with unacceptable amounts of free standing water. Moreover, an understanding of the interrelations between the waste characteristics and internal container piping was not developed. As a consequence, compliance with the free standing water requirements of 10 C.F.R. Part 61 for ion exchange resins and other liquid treatment media cannot be assured with prior art dewatering systems.